Several applications in quantum information processing, such as quantum repeaters and quantum networks, require the quantum state of a photon to be mapped onto a stable quantum memory in a reversible manner.
A semiconductor quantum dot functions as an effective quantum interface between light and matter where the polarisation of a single photon can be transcribed into the spin-state of a photogenerated exciton with high fidelity. The exciton has two spin eigenstates which correspond to two orthogonal photon polarisations, these spin eigenstates are separated in energy by the so called fine-structure splitting. The exciton can be initialised in one of the two spin eigenstates or in any superposition of these states. Thus, the exciton can be seen as a qubit.